Plutonium heat source

ABSTRACT

A radioisotopic heat source for a thermoelectric generator having a plutonium-238 alloy enclosed within a tantalum container and an arc melting method and apparatus to produce said alloy.

Elnted States Patent Inventors Donald P. Kelly Dayton, Ohio; James A. Powers, Rockville, Md.; Philip A. Tucker, Dayton, Ohio Appl. No. 616,422

Filed Feb. 3, 1967 Patented Aug. 17, 1971 Assignee The United States of America as represented by the United States Atomic Energy Commission PLUTONIUM HEAT SOURCE 4 Claims, 5 Drawing Figs.

U.S. Cl 250/106 S,

75/122.7, 136/202,252/301.1 Int. Cl G21h 5/00 Field of Search 252/301.1;

Primary ExaminerCarl D. Quarforth Assistant Examiner-R. L. Tate Attorney- Roland A. Anderson ABSTRACT: A radioisotopic heat source for a thermoelectric generator having a plutonium-238 alloy enclosed within a tantalum container and an arc meltingmethod and apparatus to produce said alloy.

TEMP. C

6-Pu+-Pu RETROGRADE MELTING PLUTONIUM HEAT SOURCE BACKGROUND OF THE INVENTION One of the uses for plutonium-238 is as an isotopic power fuel for thermoelectric generators. Typically, a quantity of plutonium-238 may be enclosed within a sealed tantalum container or cell. The cell may then be used as a heat source for a thermoelectric generator. The use of the plutonium-238 isotope as a heat source is generally limited to temperatures below the melting point of the isotope because molten plutonium reacts with the tantalum container and may result in catastrophic failure by'corrosion. Since the melting point of pure plutonium is about 640 C., pluotonium-238 heat source temperatures are generally restricted to below that point. However, higher temperatures are highly desirable because the electrical output of thermoelectric devices increases approximately exponentially as the temperature difference increases between hot and cold junctions in the assembly.

Although the melting point of pure plutonium is about 640 C., small amounts of impurities, such as iron, chromium, and nickel, which may be present with the isotope from the processing, have an adverse effect on the melting point: They form eutectics with the plutonium metal which may lower the solidus temperature by 200 C. or more. This, of course, lowers the useful temperature of the heat source due to the corrosive nature of the molten portion of the plutonium-impurity alloy to container materials.

SUMMARY OF THE INVENTION It is an object of this invention to provide an improved heat source.

It is another object of this invention to provide improved heat source alloys.

It is another object of this invention to increase the useful operating temperature in a plutonium alloy heat source.

It is another object of this invention to provide a plutonium alloy with inhibited corrosive effects against tantalum.

Various other objects and advantages will appear from the following description of one embodiment of the invention, and the most novel features will be particularly pointed out hereinafter in connection with the appended claims.

This invention comprises a mass of radioactive material composed of an alloy of plutonium-238 and zirconium enclosed within a sealed tantalum container to form a radioactive heat source for use in thermoelectric generators.

DESCRIPTION OF THE DRAWING The present invention is described in the accompanying drawings wherein:

FIG. I is a plutonium-iron phase diagram;

FIG. 2 is a plutonium-zirconium phase diagram;

FIG. 3 is a sectional view of apparatus used to prepare the heat source alloys;

FIG. 4 is an enlargement of a portion of the apparatus shown in FIG. 3; and

FIG. 5 is a sectional view a thermoelectric heat source made in accordance with this invention.

DESCRIPTION OF INVENTION Plutonium-238 has two primary disadvantages as a heat source, as previously discussed: (1) It generally contains impurities such as iron, chromium, and nickel, which lower its melting point, and (2) In its molten state it is highly corrosive to container material, such as tantalum. The first disadvantage lowers the useful or practical operating temperature of plutonium-238 as a heat source and the second disadvantage imposes a practical limitation on the temperature, and thus the efficiency, at which the heat source may operate. It may be seen that the two primary disadvantages are actually very closely related. Analyses have shown that refined plutonium- 238 may contain about two atom percent impurities. These impurities consist primarily of iron, with lesser amounts of chromium and nickel also included. A significant quantity of the iron may be removed by purifying the plutonium by ion exchange techniques. This reduction in impurity content raises the operating temperature of the heat source by raising the melting point of the plutonium and also by increasing the effectiveness of an additive (as will be discussed infra).

In FIG. 1, the plutonium-iron binary phase diagram shows the effects of relatively small percentages of iron on the melting point of a mixture of plutonium and iron..The solidus line of plutonium-iron mixture drops rapidly from the melting point of pure plutonium. One-half atom percent of iron lowers the melting point about 25, from 640 C. to about 615 C. One atom percent of iron lowers the melting point to about 585 C., and two atom percent lowers the melting point to about 540 C. or about C. below the melting point of pure plutonium. Another problem of iron as an impurity in plutonium is also shown in FIG. 1. Plutonium-iron alloys containing up to about two atom percent iron exhibit the property known as retrograde melting. After having completely solidified on cooling, alloys in this composition range will arrive at a lower temperature at which a portion of the alloy will melt. This retrograde melting temperature is about 426 C. and the solidus temperature is about 41 1 C. Thus the retrograde melting zone comprises a temperature range of only about 15, but it is obvious from previous discussion that partial melting of a heat source alloy could have adverse corrosive effects upon a tantalum container. It may be seen that even a small amount of the iron thus would severely impair the efficiency of plutonium as a heat source.

The applicants have discovered that the alloying of zirconium with plutonium-238 not only raises the solidus temperature of the alloy, but also inhibits the formation of low melting eutectics of plutonium and the impurities therein. This raises the effective temperature of the heat source enabling greater electrical output in a thermoelectric generator and inhibits corrosion of the tantalum heat source container.

FIG. 2 illustrates the high temperature portion of the plutonium-zirconium binary phase diagram. It may be seen from the diagram that the addition of about 20 atom percent zirconium may raise the melting point of plutonium about from about 640 C. to about 775 C.

The addition of zirconium has the effect not only of raising the melting temperature of plutonium, but also of inhibiting the adverse effects of iron, nickel, etc., in a plutonium alloy. Differential thermal analysis of a ternary alloy of 78 atom percent plutonium, 2 atom percent iron, and 20 atom percent zirconium does not indicate the 41 1 C. eutectic reaction characteristic of plutonium-iron binary compositions. The alloy has a liquidus temperature above 800 C. and a solidus temperature at about 640 C. although there is only a trace of of liquid present between 640 and 750 C. This appears to indicate that such an alloy may be suitable as a heat source up to about 750 C. Without any iron, the alloy melts over a temperature range of from about 815 C. to about 950 C.

Liquid zirconium, like liquid plutonium, vigorously attacks or reacts with the tantalum used as containers for the isotopic fuel, thus causing the containers to fail. However, a combination of plutonium and zirconium produces a solid which does not attack the tantalum.

By increasing the percentage of zirconium, an alloy of plutonium and zirconium may be used as a heat source at a temperature of 1000 C. In fact any alloy composition of plutonium and zirconium may be used as a heat source alloy. For example, an alloy containing about 67 atom percent zirconium has a solidus temperature above 1200 C., and the alloy could be used safely up to that temperature. However, above about 1000 C. it appears that a ceramic material may be more advantageous than a plutonium-zirconium alloy. But throughout virtually the entire temperature range at which isotopic heat sources may be used, the alloy has the advantage of good thermal conductivity over ceramic heat sources. The thermal conductivity of the alloy provides a lower temperature gradient from the center of the heat source to the container walls.

The plutonium-zirconium heat source alloy may be prepared in a commercially available arc melting apparatus such as shown in FIGS. 3 and 4. In FIG. 3, a glass bell jar may be sealingly affixed to a hearth 12, which may be a copper, water cooled hearth having an inlet 14 and an outlet 16 for the coolant. Centrally disposed within the hearth may be valved line 20 which may be connected to a vacuum pump for exhausting the bell jar and to a source of an ionizable, inert gas, such as argon, for backfilling the bell jar 10.

The hearth may contain indentations 24 and 26 each of which may be about three-fourths inch in diameter and about three-eighths inch in depth and in which may be placed, respectively, a measured amount of plutonium and zirconium and a measured amount of zirconium. An electrode 28 having a tungsten tip 30 and a handle 32 may extend into the bell jar through an aperture and through a flexible sleeve 34. Electrode 28 may be cooled with water (not shown) in a manner well known in the art. The sleeve may provide a vacuum seal for the aperture. A power supply 36 may provide the current required to produce an arc. The tungsten electrode may comprise the negative electrode in the circuit and the hearth may be positive with respect to the electrode, as shown.

After the desired quantity (and ratio) of plutonium and zirconium is placed in the indentation 24 and a desired quantity of zirconium, which may be used as a getter, is placed in the other indentation 26, the bell jar 10 and the copper hearth may be joined and sealed and a vacuum system (not shown) may be activated to evacuate the chamber within the jar. After the chamber has been evacuated to substantially remove the atmospheric gases therein, it may be backfilled through the valved exhaust and supply line 20 with an ionizable, inert gas, such as argon. The chamber may then be evacuated and may be backfilled several times, if desired, to insure an inert atmosphere devoid of active gases. The final pressure may be about three-fourths of an atmosphere, or enough pressure to allow ionization of the gas in order to sustain an arc.

After the atmosphere within the chamber has been stabilized at the desired pressure, the negatively charged electrode 28 may be activated by power supply 36 and may thereby ionize the argon gas to form an arc. The are may provide sufficient heat to melt the metals in the indentations 24 and 26 in the positively charged hearth 12. The are may be employed first to melt the zirconium in indentation 26. The zirconium is thus used to getter any trace of oxygen or other impurity from the argon atmosphere.

The are may then be changed to the plutonium-zirconium mixture in indentation 24. The electric field surrounding the tungsten tip 30 of the electrode 28 may cause the melted alloy to spin which enhances homogenization of the alloy. The circulation of coolant water through inlet 14, within the hearth, and exiting outlet 16, conducts heat away from the hearth. Due to the rapid heat transfer from sample to hearth only about 80 percent of the mixture may be melted at any one time. After the top portion, the 80 percent referred to in the preceding paragraph, has been melted and mixed, the power may be turned off and the button" may be turned over. FIG. 4, which comprises an enlarged view of indentation 24, metal button 25, and tip 30, shows an are 31 which may extend between tip 30 and the metal button in the indentation. During the period of melting and mixing the metal comprises substantially a molten globule 25 within the indentation. When the current is turned off the molten metal may rapidly solidify due to the relatively efficient transfer of heat to the copper hearth 12. Upon solidifying the molten metal may adhere to the solid, unmixed or unalloyed metal. The electrode may then be used to invert or flip the button 25 within the indentation 24 so as to superpose the unalloyed portion of the plutonium-zirconium mixture and thus expose it to a subsequent arc.

The melting and mixing cycle may be started by again activating the power supply 36 to strike an arc between the tungsten tip and the copper hearth. The arc may then be moved to the zirconium in indentation 26 to again getter any impurities from the atmosphere within the bell jar or chamber 10. The are 31 may then be moved to button 25 and may melt and mix the top percent thereof, which, of course, includes both that portion unalloyed from the prior melting and a portion of the homogenized alloy.

These steps, the gettering and the inverting and melting of the button, may be repeated as many times as desired to insure that the metal button 25 comprises a thoroughly homogenized alloy.

It appears that the quantity of metal that may be placed in an indentation may be limited by the practical realities of the electric power required to sustain an arc to melt the metal mixture. Since the power requirements are high to melt and cast, a single button of the size required for a heat source may not be preferable; it has seemed to be more practical to prepare several smaller buttons as previously described and to later cast them into the desired configuration. A single button, of course, could be prepared if desired.

If, however, one preferred to prepare several small button rather than one large one, the small buttons could be melted together and cast into the proper cconfiguration by one of several techniques. An indentation could be made on the hearth in which several buttons prepared by the arc melting technique could be cast into a long bar by raising the temperature to above the melting point. The buttons could also be cast by remelting under vacuum in a refractory crucible of the desired dimensions. Another method may be to place several buttons in their tantalum container and to apply heat under vacuum as by radio frequency techniques until the buttons fuse. Any of these methods may leave substantial voids within the configured fuel. But rather than being objectionable, the voids may be beneficial and may be desired since about a 50 void space is required to absorb helium-4 which results from the alpha decay of the fuel.

' If the alloy has not been prepared within the container as discussed in one of the examples in the preceding paragraph, it may be placed therein after fabrication has been completed. In FIG. 5 is shown a tantalum container or cell 40, which may be of cylindrical configuration, enclosing a mass 41 of radioactive material made in accordance with this invention leaving a void 43. The container may be sealed by welding a lid or cover 42 thereon in an inert atmosphere.

The use of a plutonium-zirconium alloy as herein disclosed in a tantalum container provides an improved heat source and the alloy of plutonium and zirconium raises the effective temperature at which the heat source may be used by raising the melting point of plutonium. MOreover, the addition of zirconium to plutonium inhibits the adverse effects of iron and other impurities in plutonium. Thus, the corrosion problem of molten plutonium, which has heretofore restricted the useful temperature range of plutonium in a tantalum container, is alleviated by the heat source as herein disclosed. It will be clear to those skilled in the art that any material or combinations thereof can be alloyed with plutonium-238 in accordance with this invention which results in an increase in the melting point of the radioactive material without providing any other detrimental effects. Such materials may be aluminum, gallium, lead and zinc.

It will be also understood that other changes in the details, materials and arrangements of the parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principles and scope of the invention as expressed in the appended claims.

What is claimed is:

l. A radioisotopic heat source for use in a thermoelectric generator comprising;

a. a mass of radioactive material comprising an alloy of plutonium-238 and zirconium, and

b. a sealed tantalum container enclosing said mass.

2. The radioisotopic heat source of claim 1 in which the zirconium content is about 20 atom percent.

3. The radioisotopic heat source of claim 1 in which the plutonium-238 contains not more than about 2 atom percent impurities.

4. The radioisotopic heat source of claim 1 in which said alloy mass fills only a portion of said container. 

2. The radioisotopic heat source of claim 1 in which the zirconium content is about 20 atom percent.
 3. The radioisotopic heat source of claim 1 in which the plutonium-238 contains not more than about 2 atom percent impurities.
 4. The radioisotopic heat source of claim 1 in which said alloy mass fills only a portion of said container. 